Basic receptor mechanisms the campaniform organ

Campaniform organs detect strains in the cuticular exoskeleton of insects and so provide information about loading on different parts of the body. They occur in groups at those parts where stresses are most likely to be felt, particularly near joints between segments (Fig. 4.1a, b). Mechanoreceptors that detect the relative positions of, or stresses and strains in, different parts of the body are known collectively as proprioceptors. The muscle receptor organ in the crayfish abdomen (see section 3.4) is another example of a proprioceptor, and the way that a proprioceptor influences the output from motor neurons often fits well the description of feedback in a control system (see section 1.6). The campaniform organs that occur near the joints of cockroach legs have been studied in detail, and play a significant role in normal walking movements. They detect the strains that occur when the leg is being applied to the ground and used to thrust the animal forwards. The response of the campaniform organs initiates a reflex that prevents the leg being lifted and swung forwards while it is still bearing a load (Zil! & Moran, 1981).

Sense Organs Cheetah

Figure 4.1 A simple sense organ: the campaniform organ of insects. (a) A leg of a cockroach (Periplaneta), showing the location of the groups of campaniform sensilla (arrow) on the upper surface of the tibia. (b) The surface of the tibia viewed from the direction of the arrow in (a), showing the distribution of the caps of the campaniform organs. The arrow indicates the longitudinal axis of the leg, pointing towards the body. (c) Transverse section through a campaniform organ, showing the relation of the sensory neuron to the cuticular cap and to the accessory cells, as revealed by electron microscopy. (a and b redrawn after Zill & Moran, 1981; c redrawn after Gnatzy & Schmidt, 1971.)

Figure 4.1 A simple sense organ: the campaniform organ of insects. (a) A leg of a cockroach (Periplaneta), showing the location of the groups of campaniform sensilla (arrow) on the upper surface of the tibia. (b) The surface of the tibia viewed from the direction of the arrow in (a), showing the distribution of the caps of the campaniform organs. The arrow indicates the longitudinal axis of the leg, pointing towards the body. (c) Transverse section through a campaniform organ, showing the relation of the sensory neuron to the cuticular cap and to the accessory cells, as revealed by electron microscopy. (a and b redrawn after Zill & Moran, 1981; c redrawn after Gnatzy & Schmidt, 1971.)

The ability of a campaniform organ to detect strains arises from its specialised construction, which consists of a single sensory neuron coupled with certain accessory cells and structures (Fig. 4.1c). Opposite the axon, the sensory neuron bears a short process, the dendrite, with a specialised ending consisting of a modified ciliary structure. This structure contains numerous microtubules and is connected to a specialised region of cuticle called the cap. Seen from the external surface, each cap is oval in shape, having a central dome and a surrounding depressed region. In section, it can be seen that the cuticle of the cap is thinner than elsewhere and that the centre of the dome fits tightly around the tip of the sensory neuron. It seems clear from this structural arrangement that the cap must act as a mechanism by which the strains in the cuticle are transmitted to the ending of the sensory neuron - it must act as a mechanism for coupling the stimulus energy to the receptor cell.

The electrical activity of a campaniform organ can be studied using the kind of arrangement shown in Fig. 4.2a. A cockroach leg is secured and extracellular electrodes are arranged to record activity in the main leg nerve. When bending forces are applied to the leg with a probe, a campaniform organ responds with a vigorous discharge of spikes (Fig. 4.2b). The same response can be obtained by applying a much finer probe, with a tip diameter of only 1 to 2 ^m, directly to the cap of a single campaniform organ. Furthermore, if that particular cap is ablated with a sharp needle, the discharge is no longer recorded in response to a bending force applied to the leg, which confirms the role of the cap in coupling the stimulus to the receptor cell.

The sensory neuron acts as a biological transducer, converting mechanical energy that bends the leg into electrical energy that generates the spikes. This basic process of transduction is a defining feature of all receptor cells. The precise mechanism of transduction of the sensory stimulus is only partly understood in most cases, mainly because receptor sites are small and the events occur rapidly. But it is not hard to see that in a mech-anoreceptor like the campaniform organ, mechanical distortion of the receptor cell membrane caused by the stimulus could open ion channels in the membrane. This will allow particular ions to pass through the cell membrane, resulting in a change in membrane potential. In campani-form organs, transduction probably occurs in the ciliary region of the den-drite. The dendrite lies in a lumen which almost certainly contains a special

8o Capturing sensory information (a)

8o Capturing sensory information (a)

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Figure 4.2 Physiological recording from the tibial campaniform organs.

(a) Arrangement for recording spikes in response to controlled stimuli. The stimuli are applied to the distal end of the tibia with a probe attached to a piezoelectric crystal, which bends when a voltage is applied to it. Two fine pins act as electrodes for recording from the leg nerve, and are attached to an amplifier before the signal is displayed on an oscilloscope screen.

(b) Response to bending the tibia in a ventral direction. The upper trace is a record of the voltage applied to the crystal to bend the tibia, and the lower trace is the extracellular recording of the sensory spikes. (a redrawn after Spinola & Chapman, 1975; b modified after Zill & Moran, 1981.)

Figure 4.2 Physiological recording from the tibial campaniform organs.

(a) Arrangement for recording spikes in response to controlled stimuli. The stimuli are applied to the distal end of the tibia with a probe attached to a piezoelectric crystal, which bends when a voltage is applied to it. Two fine pins act as electrodes for recording from the leg nerve, and are attached to an amplifier before the signal is displayed on an oscilloscope screen.

(b) Response to bending the tibia in a ventral direction. The upper trace is a record of the voltage applied to the crystal to bend the tibia, and the lower trace is the extracellular recording of the sensory spikes. (a redrawn after Spinola & Chapman, 1975; b modified after Zill & Moran, 1981.)

high-potassium solution, a feature that is commonly found in mechano-sensitive organs, including mammalian ears. The accessory cells are probably responsible for regulating the composition of this lumen.

The change in membrane potential that occurs at the site of transduction is known as the receptor potential. In another mechanoreceptor of the cockroach leg, a special spine near the knee, the receptor potential has been measured directly with an intracellular microelectrode (Basarsky & French, 1991), but less direct means have been used to record campaniform organ receptor potentials. The result of such an experiment is shown in Fig. 4.3, where the record of membrane potential has been redrawn as if it were recorded intracellularly. In this example, the fine stimulus probe applied to the cap was driven sinusoidally, which is an effective way to stimulate a receptor cell because the amplitude and frequency of the sine wave are

Figure 4.3 The response of a single campaniform sensillum to sinusoidally modulated indentation of its cap with a fine probe. The top trace is the monitor applied to the probe, with upward movement indicating increased force. The bottom trace shows spikes recorded with pin electrodes, as in Fig. 4.2. The middle trace indicates the appearance of the receptor potential as if it had been recorded with an intracellular electrode; upward movement of the trace indicates depolarisation of the cell membrane. This trace has been redrawn from a record of the receptor potential recorded extracellularly. (Modified after Mann & Chapman, 1975.)

Figure 4.3 The response of a single campaniform sensillum to sinusoidally modulated indentation of its cap with a fine probe. The top trace is the monitor applied to the probe, with upward movement indicating increased force. The bottom trace shows spikes recorded with pin electrodes, as in Fig. 4.2. The middle trace indicates the appearance of the receptor potential as if it had been recorded with an intracellular electrode; upward movement of the trace indicates depolarisation of the cell membrane. This trace has been redrawn from a record of the receptor potential recorded extracellularly. (Modified after Mann & Chapman, 1975.)

readily varied and the resulting responses lend themselves to quantitative analysis.

The sinusoidal stimulus elicits a large receptor potential, which follows the imposed force closely: as the force indenting the cap increases, the cell becomes increasingly depolarised; and as the force decreases the cell becomes repolarised (middle trace in Fig. 4.3). Spikes are generated by the depolarisation and are superimposed on the basic shift in membrane potential; they are propagated along the axon to the central nervous system. The change from receptor potential to spikes is termed coding because the analogue signal of the receptor potential is converted into the digital code of spike frequency, which is used for long-distance communication in the nervous system. The conversion of receptor potential into spikes occurs at the place where the axon originates from the cell body, the spike-initiation zone. The mechanism by which it occurs is the same as that by which spikes are triggered by summation of excitatory synaptic potentials in other neurons (see Chapter 2). Transduction of an increasing stimulus will cause a greater current flow and so depolarisation of the membrane at the spike-initiating zone will occur more rapidly. This means that the time interval between successive spikes will be less, and so the spike frequency will be higher.

Inspection of Fig. 4.3 shows that, although the receptor potential mirrors the changing stimulus rather faithfully, the train of spikes reflects the stimulus pattern less satisfactorily. The axon must employ spikes for communication because the graded receptor potential could not travel far, being limited by cable properties of the neuron. However, the change from an analogue to a digital code involves a certain loss of information because the use of a frequency code means that both encoding and decoding the signal require time - a particular stimulus strength is immediately represented by a particular receptor potential, but requires at least two separate spikes to encode it. The closer together the spikes are in time, the stronger the stimulus.

In addition to the intrinsic limitations of coding, there is another process at work in shaping the response of the sensory neuron. Looking back to Fig. 4.2b, when a constant bending force is first applied, there is a vigorous response to the onset of the stimulus but spike frequency then soon declines. Similarly, in Fig. 4.3, spike frequency initially rises with increasing stimulation, but begins to decline even before the stimulating force has reached its maximum value. This pattern of response, whereby a receptor cell responds vigorously to a changing stimulus but soon ceases to respond to a steady stimulus, is known as adaptation and is a property of most sensory cells. Generally, changes in stimulus strength are more significant than constant stimuli, and adaptation is a means of enhancing the detection of changing stimuli.

A sensory neuron that is much more sensitive to a rapid change in the stimulus is said to be quickly adapting or phasic; one that is sensitive to slowly changing or maintained stimulation is said to be slowly adapting or tonic. Naturally, phasic and tonic receptor cells provide an animal with information about different types of situation. A phasic receptor provides information about rapidly changing events, whereas a tonic receptor keeps an animal informed about a steady background situation. Many sensory neurons adapt to a stimulus in a way that represents some combination of phasic and tonic properties. The campaniform organ is an example of this because it is most sensitive to a rapid increase in applied force, but it also exhibits a declining response to a maintained stimulus.

The tibial campaniform organs in the cockroach leg are arranged in two subgroups with mutually perpendicular cap orientations (see Fig. 4.1b). The longitudinal axes of the caps of the distal group lie parallel to the longitudinal axis of the tibia. In physiological recordings, it is found that campaniform organs in the distal group respond only to downward bending of the tibia and those in the proximal group respond only to upward bending. This pattern of response can be understood by thinking of the tibia as a cardboard tube and considering the effects of bending in one direction and then in the opposite. The upper surface of the tibia, where the campaniform organs are located, will undergo transverse compression when the tibia is bent down and longitudinal compression when it is bent up. Hence, the fact that the distal campaniform organs respond to downward bending and the proximal ones to upward bending shows that each sensory neuron is sensitive only to transverse compression of its cap (Zill & Moran, 1981).

The tibial campaniform organs, therefore, act as directionally sensitive strain gauges and the two groups of campaniform organs below the knee provide the animal with information about opposing forces to which the tibia is subjected during walking. The organs are most sensitive to bending forces in the plane of the femoro-tibial joint, whether these are produced by external factors or by contraction of the muscles that move the tibia. Forces that bend the leg at unnatural angles or other forces, such as those twisting it about its longitudinal axis, produce little or no response in any of the campaniform organs. The local forces of compression that best excite a particular campaniform organ constitute its receptive field, a concept previously introduced in the description of the wind-sensitive giant interneurons of the cockroach (see Chapter 3).

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